Manufacture of phosphoric acid



Aug. 15, 1939. J E, MALQWAN 2,169,588

MANUFACTURE OF PHOSPHORIC ACID Filed Dec. 23, 1-956 Aci tl Slim-age Jolui E. Mulowar u INVEANT-OR ATTORN EY Patented Aug. 15, 1939 UNITED STATES MANUFACTURE OF PHOSPHORIC ACID John E. Malowan, Dayton, Ohio, assignor to Monsanto Chemical Company, a corporation of Delaware Application December 23, 1936, Serial No. 117,257

4 Claims.

This invention relates to a process for the manufacture of phosphoric acid.

One of the objects of the present invention is the provision of a process for the manufacture of strong phosphoric acids. A further object is the provision of an efficient continuous process for the manufacture of phosphoric acid. by the hydration of phosphoric anhydride.

If phosphoric anhydride, P205, be added to water, a vigorous reaction occurs so that a portion of the light fluffy anhydride is expelled by the steam formed, resulting in a loss to the process. Strong orthophosphoric acids up to 100% H3PO4 may be employed in place of water for the hydration of phosphoric anhydride, and the violence of the reaction considerably mitigated. Acids of such strengths, especially when heated to temperatures of to 150 0., which may occur in a continuously operated process, have an appreciable water vapor pressure and it is therefore difficult to introduce the phosphoric anhydride by mechanical conveying means, in atmospheres charged with such appreciable quancities of water vapor.

I have now discovered that if the phosphoric anhydride be added to phosphoric acids of strengths above H3PO4; i. e., having a P205 content of more than 72.43%, I am enabled to effect a rapid and complete solution of the anhydride under conditions of temperature such that rapid solubility is attained and also under conditions where water vapor is either substantially absent or has been reduced to such a point that its presence is no longer a source of difficulty in the operation of the process. I also avoid the formation of difficultly soluble lumps of phosphoric anhydride or of partly hydrated anhydride. The process is thus capable of producing a pure concentrated phosphoric acid of any strength up to a maximum of about 84% P205 concentration which upper limit should not be exceeded because of the excessively high viscosity of acid of higher concentration.

My process will be explained by reference to the accompanying drawing, the single figure-of which illustrates one way by which my process may be carried out.

Chamber Ill which may be a simple iron box provided with a hopper bottom II is connected to a phosphorous burner by conduit l2. The phosphorus burner which is not shown, may be of any approved type in which phosphorus is burned in an excess of previously dried air, and supplies the powdery phosphoric anhydride, suspended in air to the conduit 12. The powdery anhydride, upon reaching the enlarged chamber Ill settles out substantially completely, the waste gases passing out of chamber ill by means of pipe I3. The anhydride collects in the hopper ll of chamber ill and by means of conveyor [4, 5 working within conveyor shell I5, is conveyed from hopper II to a mixing device [6, which in the present embodiment takes the form of a ball mill constructed of acid proof material. The ball mill has previously been charged with strong 10 phosphoric acid of such strength as has been found workable in my present invention. The ball mill is rotated, and as the anhydride is mechanically introduced by the screw I l a stream of acid of strength such as is hereinafter set 15 forth is introduced by means of acid supply pipe IT. The temperature of the acid in the ball mill or such other mixing device as may be employed is maintained within the range of 65 to (3., by means of cooling water applied as at 29 and 20 30 under which conditions rapid and complete solubility is effected by the agitation caused by the mixing device.

The acid supplied by pipe I! and the anhydride supplied by the screw M are both adjusted to 5 the point where an acid having a strength of from 78% to 84% P205 content, equivalent to 108% to 116% I-I3PO4 content, is discharged from the mill 16 at the discharge port l8. The discharged acid from 18 is collected in 30 tank H), from which a portion thereof is con-- veyed to tank 22 by pipes 20 and 2|. Water is then added as by pipe 23 to the acid in tank 22 and the dilution carried to the point where the acid. has a concentration from 74% to 82% P205 35' (102% to 113% H3PO4 equivalent). The upper limit here given may be extended to a concentration approaching that finally desired. The diluted acid is' then returned by means of pipe I! to the mixing chamber IS.

The upper concentration of the recycled acid, that is the acid produced by dilution of strong acid in tank 22 and then delivered by means of pipe H to the mixing device is of course, not limited to the concentrations set out in the above 45 example. The upper concentration of such acid may approach that of the acid finally produced. Considerable heat is developed during dilution and I find it convenient to remove such heat by cooling water which is introduced by means of 50 siderable variation in the type of apparatus employed as is herein noted. In place of the screw conveyor I4, I may employ other feeding means such as magnetically operated powder feeders, or other mechanical devices. Mixing device l6 which may be operated with or without balls or rods is employed as a mixing and cooling device and may be replaced by a covered tank equipped withcooling coils and containing a mechanically driven stirrer. It is of course understood that all parts coming into contact with the acids are constructed of acid proof material such as enameled iron, ceramic materials or stainless steel.

The temperature and concentration ranges herein set out are critical Within the limits given for the following reasons:

Between temperatures of 65 and 140 C acid having a concentration of 74% to 82% P205 (102% to 113% H3PO4 equivalent), the lower temperature relating to the lower concentration, and the higher temperature to the higher concentration, the powdery anhydride may be dissolved at a practical rate by means oi ordinary mixing equipment. At concentrations higher than 84% P205 the viscosity even at elevated temperatures is so high that mixing of the dry anhydride is very slow and diflicult. For the same reason cooling of the acid during mixing is more difficult. At temperatures lower than those stated I find the rate of solution to be unduly slow, At and between these temperature limits and within the acid concentration stated, the lower temperature applying to the lower concentration, the higher temperature to the higher concentration; I find that very little water vapor exists in the at- .mosphere above these acids and hence I do not encounter a premature hydration of the anhydride before it is mixed into the body of the acid. I have found that if such premature hydration takes place either during the manufacture of the anhydride itself or at the end of the feeding device, exemplified in the drawing by the feed screw l4, a difiicultly soluble formof meta phosphoric acid is formed. If such formation does take place it will eventually stop the operation of the anhydride feeding device, and hence interfere with the continuous carrying out of the process.

As a result of operating within the temperatures and concentrations herein specified I find that I obtain thereby an important elimination of fluorine. Experiments have shown that approximately to of the fluorine is eliminated during hydration of the anhydride. Since'anhydride usually contains from 30 to 50 parts per million of fluorine, acid having from 3 to 6 parts may be easily produced, thus making unnecessary any further treatment for fluorine elimination,

Analysis of the strong phosphoric acids'used or produced by the herein described process show that appreciable quantities of meta-phosphoric acid are present therein. The following figures are cited to show the composition of the acid as existing at room temperatures, that is at the temperature at which the analyses were made:

Total Since the relationship between the three acids shown is known to change with temperature, the above figures are not indicative of the composition at elevated temperature.

The principles of my invention herein enunciated, while being particularly applicable to the continuous manufacture of phosphoric acid by the hydration of phosphoric anhydride, are or" equal benefit when operating in a batch wise manner,

Having now particularly described my invention and the manner in which it may be carried out practically, I desire that my invention be not limited except as required by the prior art, or as particularly set forth in the appended claims.

What I claim is:

1. A process for manufacturing strong phosphoric acid comprising dissolving substantially dry, solid phosphoric anhydride in a phosphoric acid having a strength of from 74% to 82% P205, while maintained at a temperature of from 65 C. to C, r

2. Aprocess for manufacturing phosphoric acid having a concentration up to a maximum of 34% P205, comprising dissolving substantially dry, solid phosphoric anhydride in a phosphoric acid having a strength between the limits of 74%' to 82% P205, while maintainingthe temperature of said dissolving acid between 65C. and 140 0;,

diluting said acid to a strength of between 74% and 82% P205 content and'again adding phosphoric anhydride to said diluted acid, 5

3. A continuous process for manufacturing 7' phosphoric acid having a centrationbetween 78% acid of strength between 78% and 84% P205, and

diluting the remainder of said acid to a concentration of between 74% and 82% P205 and again adding phosphoric anhydride thereto.

4. A continuous process for the manufacture of phosphoric acid comprising mixing substantially dry, solid phosphoric anhydride and phosphoric acid of a strength between 74% and 82% P205,

maintaining the temperature of said acid duringmixing between 65 C. and 140 C. said lower temperature corresponding to said lower concentration and said higher temperature corresponding to said higher concentration, and proportion ing the relative rates of flow of said acid and anhydride to produce an acid having a concentration of from 78% to 84% P205.

JOHN E. MALOWAN. 

